Silicon nitride, sialon and similar ceramics characteristically not only have a smaller specific gravity and are more corrosive-resistant than steel but are also insulating. Accordingly, if ceramics are adopted as a source material for a component of rolling bearings (including hub units) including a race member and a rolling element, or a component of a universal joint including a race member and a torque transmission member, such as a race member, a rolling element, a torque transmission member, they can provide bearings, universal joints and the like reduced in weight and also prevent rolling bearings and universal joints from having short life as their components corrode and are thus damaged or electrolytically corrode.
Furthermore, a hub unit, which is a type of rolling bearing, is often used in an environment having a possibility of receiving moisture therein and hence having insufficient lubricity. Ceramic rolling elements, race members and similar rolling contact members are characteristically less damageable in such an insufficiently lubricating environment as above. Accordingly, for example, a hub unit with a rolling contact member formed with ceramics adopted as a source material can exhibit improved durability when it is employed in an insufficiently lubricating environment.
Furthermore, a universal joint has a torque transmission member rolling and stopping repeatedly on a surface of a race member, and between the torque transmission member and the race member there is not sufficient oil film provided. Furthermore, a universal joint is often used in an environment having a possibility of receiving moisture therein and hence having insufficient lubricity. A ceramic torque transmission member is characteristically less damageable in such an insufficiently lubricating environment as above. Accordingly, for example, a universal joint with a torque transmission member formed with ceramics adopted as a source material can exhibit improved durability when it is employed in an insufficiently lubricating environment.
However, silicon nitride, sialon and similar ceramics require higher production cost than steel, and adopting ceramics as a source material for components of rolling bearings and universal joints disadvantageously increases their production costs.
In recent years, there has been developed a method of producing β-sialon, a type of ceramics, inexpensively by adopting a production process including combustion synthesis (Japanese Patent Laying-open No. 2004-91272 (Patent Document 1), Japanese Patent Laying-open No. 2005-75652 (Patent Document 2) and Japanese Patent Laying-open No. 2005-194154 (Patent Document 3)). This allows one to consider adopting β-sialon as a source material for components of rolling bearings, universal joints and the like to produce them inexpensively.
Patent Document 1: Japanese Patent Laying-open No. 2004-91272
Patent Document 2: Japanese Patent Laying-open No. 2005-75652
Patent Document 3: Japanese Patent Laying-open No. 2005-194154